Bad Wolf is a message from Rose Tyler, an old freind of the doctors which means its the end of the world. We first saw it in the unquiet dead, then Dalek and then we saw a boy painting those words on the back of the TARDIS in Aliens of london.
In the long game, the face of boe had a baby called boemina, this was announced on sattilite five, BadWolfTV.
In Bad Wolf the walls of sattelite five said Bad Wolf coroporation
We also saw it drawn on the concrete in the parting of the ways
In Turn Left, Bad Wolf was written everywhere, even the TARDIS!